1. Field of the Invention
This invention relates generally to disk storage for computer systems. In particular, it is directed to an array of synchronous disk drives that emulate a single logical disk drive.
2. Description of the Prior Art
Disk drives have long been popular mass storage devices. They provide a low cost solution to the problem of non-volatile data storage. Virtually all computer system manufacturers, therefore, provide for disk drives as system peripherals.
The major advantage of disk drives is low cost. This advantage is outweighed for some applications by the disadvantage of insufficient data transfer speed, particularly in super computer and other high performance computing environments such as the type manufactured by Cray Research, Inc., the assignee of the present invention. The problems facing a computer system user wishing to increase the data transfer rates of disk drives are not trivial. Up until now, most solutions have sought to incrementally enhance the performance of a single disk drive while retaining the disk drive's basic architecture.
The basic structure of the disk drive consists of a metal disk coated with magnetic material rotating under one or more read/write heads. Most disk drives are multi-platen systems where a number of the metal disks are arranged in a stack.
All data transfers to disk drives are sequential in the sense that data moves in or out sequentially one word at a time. The access time to a selected word is partially dependent on its location. Data is recorded on the disk in concentric circles called "tracks". The disk drive has detection means for indicating when the magnetic head is positioned at the outermost track. A stepper motor (or servo-controlled linear motor) controls the head position causing it to step from track to track. This head positioning function is called a "seek". The period required to position the Read/Write heads from the time the command is received until the time the drive becomes ready is known as the seek time.
Once a track is selected, it is necessary to wait for the desired location to rotate into position under the head. The average waiting time, known as latency time, is the time for half a revolution.
Within each track, information is organized into segments called "sectors". A sector can consist of any number of bytes, limited only by the storage capacity of the track. The addressing of sectors is typically a software function. So that the sectors can be identified by the software, each sector is preceded by an identifier block. The format of this identifier block is system dependent.
Usually each track is single bit serial, so that each byte is stored as eight consecutive bits on a track. Because track selection and latency increase access times, it is preferable to transfer large blocks of data which will be stored in sequential locations. Once the disk heads are positioned at a particular track and no further head movement is required, data will be transferred at a fixed rate. This fixed rate is determined by the speed of the disk drive and is independent of the computer system itself.
Prior art disk drives are limited in their capacity and speed in transferring data. The prior art is lacking in high performance disk drives which will allow data transfer rates exceeding the speed of currently available disk drives. The prior art disk drives are also limited in their capacity to store data. There is a need in the art, therefore, for a higher capacity disk drive than that currently available.